1. Field of the Invention
The present invention relates to a sheet feeding apparatus and an image forming apparatus, and in particular, relates to the configuration of a sheet separating and feeding portion which separates and feeds sheets one by one to an image forming portion.
2. Description of the Related Art
Recently, an image forming apparatus such as a printer, a copying machine and a facsimile machine includes a sheet feeding apparatus for separating and feeding sheets one by one to an image forming apparatus. Such a sheet feeding apparatus includes a sheet separating and feeding portion for separating and feeding the sheets one by one.
As such a sheet separating and feeding portion, a retard separation method including a feed roller which rotates in a sheet feeding direction, a separation roller which is press-contacted to the feed roller with a predetermined pressure and a torque limiter which is coupled to the separation roller is disclosed in Japanese Patent Application Laid-open No. 5-338837.
Pressing force of the separation roller against the feed roller and a torque value of the torque limiter are set so that the separation roller is rotated to be driven by the feed roller or the sheet in a case that there is no sheet at a nip portion of the feed roller and the separation roller or a case that only one sheet is nipped. On the contrary, the pressing force of the separation roller and the torque value of the torque limiter are set so that the separation roller is reversely rotated in a case that a plurality of sheets are introduced into the nip portions of the feed roller and the separation roller.
FIG. 7 is a view which illustrates the configuration of a sheet feeding apparatus in the related art including the sheet separating and feeding portion of the abovementioned retard separation method. A cassette 200 which accommodates (i.e., supports) sheets S, a downstream side-wall 203 in the sheet feeding direction of the cassette 200, an intermediate plate 201 which is arranged in the cassette 200 movably in the vertical direction are provided as illustrated in FIG. 7. The sheets S are mounted on the intermediate plate 201.
A pickup roller 129 feeds the top sheet Sa which is accommodated in the cassette 200. A retard roller 131 which is formed of ethylene-propylene-diene rubber (EPDM), for example, is press-contacted to a feed roller 130 which is formed of urethane, for example, by a spring which is not illustrated.
By press-contacting the retard roller 131 to the feed roller 130, a separation nip portion N is formed between the retard roller 131 and the feed roller 130 in a state that a surface of the retard roller 131 contacted to the feed roller 130 is compressed.
Here, the top sheet Sa sent by the pickup roller 129 is fed to the separation nip portion N which is formed with the feed roller 130 and the retard roller 131. A conveyance path 303 is formed with a conveyance guide 302 which is provided from the separation nip portion N to a pair of conveying rollers 305a, 305b. 
In the sheet feeding apparatus of the related art, in order to perform sheet feeding, first, the pickup roller 129 is rotated and the top sheet Sa accommodated in the cassette 200 is conveyed to the separation nip portion N.
Then, in the case that only one sheet Sa is conveyed to the separation nip portion N, as illustrated in FIG. 8A, the driving of the retard roller 131 is discontinued due to operation of the torque limiter and the retard roller 131 is rotated along with the sheet Sa. Thus, the sheet Sa is passing through the conveyance path 303.
On the other hand, in the case that a plurality of sheets are conveyed to the separation nip portion N, the retard roller 131 is rotated in the opposite direction to the feed roller 130 without being rotated along with the feed roller 130 due to the operation of the torque limiter. Then, since the retard roller 131 is rotated in the direction opposite to the sheet conveying direction as mentioned above, only one sheet Sa which is contacted to the feed roller 130 is conveyed to the downstream side. Then, other sheets are returned to the upstream side in the sheet conveying direction by the retard roller 131 as illustrated in FIG. 8B. In this manner, overlapped sheet feeding can be prevented.
Here, in the sheet feeding apparatus and the image forming apparatus therewith in the related art, the feed roller 130 and the retard roller 131 are to be worn as usage time of the apparatus becomes long. Then, as the abrasion increases, there arises a case that the friction force between the sheet and the retard roller and the sum of the torque of the torque limiter and the rotation resistance of the retard roller satisfy expression 1 of the following.The friction force between the sheet and the retard roller<The torque of the torque limiter+the rotation resistance of the retard roller  (Expression 1)
In this case, when the sheet is conveyed to the separation nip portion N, the retard roller 131 is rested without being rotated along with the sheet which is sent by the feed roller 130. Namely, the retard roller 131 fails to be rotated along with the sheet. Here, even in the case that the retard roller 131 fails to be rotated along with the sheet, separating and conveying of the sheet is performed if following expression 2 is satisfied when one sheet is conveyed to the separation nip portion N.The friction force between the feed roller and the sheet>The friction force between the sheet and the retard roller  (Expression 2)
Further, even in the case that two or more sheets are conveyed to the separation nip portion N, the separating and conveying of the sheet is performed if following expression 3 is satisfied.The friction force between the sheets<The friction force between the sheet and the retard roller  (Expression 3)
Further, even in the state that failure of rotation of the retard roller 131 occurs, the friction force between the retard roller 131 and the feed roller 130 is larger than the friction force between the sheet and the retard roller 131, normally. Therefore, following expression 4 is satisfied. Since the retard roller 131 is rotated before the sheet enters into the separation nip portion N, the sheet is introduced and conveyed to the separation nip portion N.The friction force between the feed roller and the retard roller>The torque of the torque limiter+the rotation resistance of the retard roller  (Expression 4)
However, the abrasion of the retard roller 131 does not occur evenly. As usage time of the apparatus increases, the abrasion of the retard roller 131 increases at a specific part. Namely, the state of expression 1 occurs not at entire circumference of the roller and the abrasion is increased a little more at a part than at the other parts. Accordingly, the frequency of stopping of the rotation thereat becomes high. Then, when the rotation is stopped, the sheet is rubbed and the abrasion at the part is rapidly increased. Thus, the retard roller 131 is worn in part and cannot be rotated along with the sheet.
Here, when the image forming apparatus is rested, the retard roller 131 is rested in a state that the specific part of which abrasion is increased is opposed to and press-contacted to the feed roller 130. In this case, a hollow is generated at the specific part due to the press-contact force with the feed roller 130. When this state continues for a long time, the hollow at the specific part becomes large and the specific part is not resumed as illustrated in FIGS. 9A and 9B.
Here, when the hollow becomes large, the rotation resistance of the retard roller 131 is increased. When the feed roller 130 is rotated in this condition, the surface of the retard roller 131 is further trimmed. Then, in the case that the abovementioned phenomenon is repeated, the abrasion is increased and the hollow becomes large only at the specific part of the retard roller 131. As a result, decrease of the left side and increase of the right side of expression 1 are accelerated only at the specific part of the retard roller 131. Accordingly, a part which satisfies the relation of following expression 5 appears prematurely.The friction force between the feed roller and the retard roller<The torque of the torque limiter+the rotation resistance of the retard roller  (Expression 5)
As illustrated in FIG. 9A, when the specific part with the increased abrasion and hollow satisfies the expression, the retard roller 131 remains rested even though the feed roller 130 is rotated. Accordingly, as illustrated in FIG. 9B, the top end of the sheet S impinges to the circumference of the rested retard roller 131 and entering of the sheet S into the separation nip portion N is obstructed so that the sheet S is jammed. In this manner, when the hollow is generated at the retard roller 131, jamming of the sheet S occurs.
The present invention provides a sheet feeding apparatus and an image forming apparatus in which occurrence of a hollow at a retard roller (i.e., a separation roller) can be prevented.